Coherent-on-receive techniques are successfully used to provide coherence in prior art pulsed radar systems. These systems use analog techniques for measuring and storing the phase relationship between the transmitted signal and a stable local oscillator and for similarly measuring the phase relationship between the received signals and the same stable local oscillator. The analog stored comparisons are then used to introduce a phase correcting factor to the received signals thereby producing coherent received signals. U.S. Pat. No. 3,680,099 issued to Elvin E. Herman and Henry F. McCord and U.S. Pat. No. 3,372,389 issued to B. M. Bellman, et al reveal the current state of the art of such analog radar systems. Introduction to Radar Systems, M. I. Skolnick, McGraw Hill Publishing Co., (1962) pp. 118-119, also depicts a system of this type. The analog techniques utilized in prior art systems for measuring and correcting the phase relationships are limited in accuracy by circuit drift and noise considerations. In U.S. Pat. No. 3,680,099, the correction is accomplished in the Intermediate Frequency (IF) portion of the receiver. In U.S. Pat. No. 3,372,389, the output of the system is doppler velocity only, the amplitude of the return signal not being presented. The system described in Introduction to Radar Systems, supra, p. 118, uses a "Coho" (coherent oscillator) which must be locked or synchronized by an IF locking pulse which is related in turn to the phase relationship between the transmitted pulse and a "STALO" (stabilized local oscillator). The phase correction is accomplished at the Intermediate or Radio Frequency. The phase locking accuracy degrades as the transmitted pulse width decreases, rendering the analog techniques unuseable in most short pulse radar applications.
These coherent-on-receive systems have been used in Moving Target Indicating Radar to provide the necessary coherence where no fixed background or clutter signal is available to provide a phase reference from pulse to pulse. They have also been used where the system output requirement is for a doppler signal indicating velocity of a moving target or the relative velocity of a fixed target. The latter case is typified in a high resolution synthetic array radar such as a side looking radar where the zero doppler component output may be used to indicate that the target is 90 degrees to the track of the radar carrying vehicle. Thus, if the doppler component of the target return is filtered in a suitable low pass or matched filter, the effective beam width of the radar antenna may be reduced, thus greatly enchancing the resolution of the system. It will be appreciated, then, that the systems described in the prior art and subsequently described herein are applicable to mobile vehicle mounted synthetic array radar systems and to moving target indicator radars whether fixed or mobile.